The semiconductor industry is characterized by a trend toward fabricating larger and more complex circuits on a given semiconductor chip. The larger and more complex circuits are achieved by reducing the size of individual devices within the circuits and spacing the devices closer together. As the dimensions of the individual components within a device such as an MOS or bipolar transistor are reduced and the device components brought closer together, improved electrical performance can be obtained. However, attention must be given to the formation of doped regions in the substrate to insure that deleterious electrical field conditions do not arise.
As the size of device components such as the transistor gate in an MOS device and the emitter region in a bipolar device, are reduced, the junction depth of doped regions formed in the semiconductor substrate must also be reduced. The formation of shallow junctions having a uniform doping profile and a high surface concentration has proven to be very difficult. A commonly used technique is to implant dopant atoms into the substrate with an ion implantation apparatus. Using ion implantation, the high energy dopant atoms bombard the surface of the substrate at high velocity and are driven into the substrate. While this method has proven effective for the formation of doped regions having moderately deep junctions, the formation of ultra-shallow junctions using ion implantation is extremely difficult. Both the path of the energized dopant atoms within the substrate and the implant uniformity are difficult to control at the low energies necessary to form shallow implanted junctions. The implantation of energized dopant atoms damages the crystal lattice in the substrate which is difficult to repair. Dislocations resulting from the lattice damage can easily spike across a shallow junction giving rise to current leakage across the junction. Moreover, the implantation of P-type dopants such as boron, which diffuse rapidly in silicon, results in excessive dispersion of dopant atoms after they are introduced into the substrate. It then becomes difficult to form a highly confined concentration of P-type dopant atoms in a specified area in the substrate and especially at the surface of the substrate.
A similar problem is encountered when a dopant such as boron is diffused from an overlying polysilicon layer directly into a silicon substrate. The boron rapidly diffuses through the polysilicon and into the substrate. The rapid introduction of boron atoms into the substrate, coupled with the high diffusivity of boron in silicon, results in excessive lateral diffusion.
The most serious consequence of the excessive dopant diffusivity is termed "lateral diffusion" wherein a diffused region extending laterally parallel with the surface of the substrate is formed. If the lateral diffusion becomes too great, the device performance will be severely degraded and in the worst case, total functional failure occurs. In P-channel MOS transistors and NPN vertical bipolar transistors, the active components which substantially determine performance factors such as current gain and switching speed are fabricated by forming shallow boron doped regions in the substrate. For example, the boron doped LDD portion of the MOS P-channel transistor must offset the drain electric field from the channel region to prevent hot carrier injection while simultaneously carrying a large quantity of charge to maintain a high current gain. In the vertical NPN bipolar transistor, the boron doped base region must have a high dopant concentration to maintain high current gain but not excessively overlap the emitter region. Excessive lateral diffusion of the base regions can result in a low emitter-base breakdown voltage and hot carrier injection along the periphery of the emitter.
In VLSI device fabrication, as dimensions become smaller, lateral dopant diffusion, especially P-type dopant diffusion, must be minimized in order to fully realize enhanced device performance. This is necessary to prevent the associated electric fields of the doped regions from overlapping and forming unwanted parasitic capacitors within the device.